Patent GB-1185630-A (GB)
Title: Improvements in and relating to Speed Governing Systems for Steam Turbines
Inventor: MITCHELL JOHN MACKENZIE
IPC: F01D17/18
Filed: 1966-11-04
Abstract: 1,185,630. Steam turbines. C.A. PARSONS & CO. Ltd. Nov.3, 1967 [Nov.4, 1966], No.49453/66. Heading F1T. [Also in Divisions Gland G3]. In a steam turbine plant comprising a high pressure (H.P.) turbine and a lower pressure (L.P.) turbine with a governing valve controlling the admission of steam to the H.P. turbine and a further valve controlling the flow of steam from the H.P. to the L.P. turbine, there is provided a speed governing system which alters the relationship between the degree of opening of the governing and further valves according to the load on the turbine. In the plant, a governing valve 1 controls the steam flow to a H.P. turbine and an intercept valve 2 controls the flow to a L.P. turbine from an inter-stage reheater. The valves are controlledby relays 3, 4, respectively, responsive to the turbine governor which transmits electric signals via a potentiometer 7 to electro-hydraulic servovalves 5, 6 to actuate the relays. The governing system is arranged such that when the plant is operating in the range between full turbine load and approximately half load the valves 1, 2 open and close by the same degree, Fig.2 (not shown). Over this load range the steam pressure in the reheater remains substantially constant, Fig. 3 (not shown). Should the turbine load be below half load, devices 8, 9 are provided to modify the electric signal to servovalve 6 to maintain the opening of valve 2 constant at the half load value whilst the opening of the valve 1 continues to vary in linear manner with load. At the same time devices 8, 9 increase the rate at which valve 2 can close in response to a given rate of change of control signal from the governor so that the valves 1, 2 reach their fully closed positions simultaneously when the turbine load is completely shed. Over this lower load range the reheater pressure varies with the load on the turbine. Device 8 may be a potentiometer acting in conjunction with a fixed voltage to produce a variable output voltage that is added to the signal from potentiometer 7, and device 9 changes the setting of a variable potentiometer 10 to alter the gain in the control line to servovalve 6. Device 9 includes a time delay to achieve the above mentioned faster closing of valve 2 with rapid load reduction. Devices 8, 9 are responsive to turbine load by way of a wattmeter associated with a generator driven by the plant, or may be responsive to reheat pressure if a device is provided to control independently the reheat pressure in response to load. An additional control device (11, Fig. 1a, not shown), may be provided to add a further voltage signal to the output of potentiometer 10 to increase the accuracy of control of the reheat pressure. To prevent unstable control of valve 2 during steadystate operation below 25% full load, control in this range may be achieved by retaining the setting of potentiometer 10'at the 25% full load value and by adding a suitable voltage through device 8. In modifications, the turbine governor may generate pressure and/or electrical signals instead of producing solely electrical signals.